1. Field of the Invention
The present invention relates to a display device including plural pixel circuits arranged in a matrix, each pixel circuit having a light-emitting element that emits light with a luminance corresponding to an amount of injected electric current and a transistor element that controls the amount of electric current flowing through the light-emitting element, the display device being formed to accumulate electric charges up to a predetermined capacitor and to detect/supply a voltage corresponding to a driving threshold voltage between a gate and a source of the transistor using the accumulated electric charges prior to the light emission by the light-emitting element. The present invention also relates to a method of driving such display device.
2. Description of the Related Art
An organic light-emitting display device which employs an organic light-emitting diode (OLED) that emits light by itself is the most appropriate device for the realization of flat screen display devices since such OLED eliminates the need of backlights required in liquid crystal displays. Further, the OLED has no restriction in viewing angle. Thus, the OLEDs attract attentions as the next-generation display device which would replace the liquid crystal display, and the practical application thereof is being waited for.
Known image display devices using the OLEDs are classified into a simple (passive) matrix type and an active matrix type. The former, though being advantageous for its simple structure, is not appropriate for realization of large high-resolution display devices. Thus in recent years, the development efforts concentrate on the active matrix type display device which controls electric currents flowing through light-emitting elements in pixels by active elements provided in the pixels, such as driver elements formed from thin film transistors (see Japanese Patent Laid-Open No. 2002-196357, for example).
FIG. 7 is a circuit diagram of a structure of a pixel circuit corresponding to a single pixel in a conventional image display device. The single pixel will be referred to as a sub-pixel for one of R, G, and B in one pixel if the display device is a color display below. As shown in FIG. 7, a pixel circuit 100 includes an OLED 101 which function as a light-emitting element, a driver element 102 which serves to determine an amount of electric current flowing through the OLED 101, a first switching element 103 which serves to control driving state of the driver element 102, a second switching element 104 and a third switching element 105 which functions at a threshold voltage detection described later, and a capacitor 106 arranged between a gate electrode and a source electrode of the driver element 102. Further, the conventional display device has a structure in which electric signals are supplied for drive control from a driver circuit 112 via a low potential supplying line 107, a high potential supplying line 108, a scan line 109, a first control line 110, a second control line 111, and a data line 113 to the pixel circuit elements described above.
The driver circuit 112 serves to supply electric signals for the control of the driving state of the elements in the pixel circuit 100. Specifically, the respective circuit elements in the pixel circuit 100 has functions such as: supplying a driving threshold voltage of the driver element 102 in advance; accumulating a predetermined amount of electric charges for the OLED 101 prior to the supply of the driving threshold voltage; supplying a potential corresponding to a gradation level of the OLED 101 to the driver element 102; and supplying a voltage between an anode and a cathode of the OLED 101 to let the OLED 101 emit light with luminance corresponding to the gradation level. The driver circuit 112 supplies predetermined electric signals via elements such as the low potential supplying line 107 to realize these functions.
The conventional display device with the OLEDs, however, has a large number of wirings extending from the driver circuit 112 as an interconnection structure, whereby the improvement in aperture ratio of each pixel is difficult to achieve. Inconveniences faced in the conventional display device will be described in detail below.
The conventional display device is structured so that the plural pixel circuits 100 are arranged in a matrix. Operations such as the supply of the driving threshold voltage by the driver element 102 are performed in each of the plural pixel circuits 100. Here, in the conventional display device, data voltage is supplied sequentially to the pixel circuits arranged in one row via a single data line 113. Then, the operations such as the supply of the driving threshold voltage is performed simultaneously to the pixel circuits 100 arranged in the same row, while such operations are performed at different timings corresponding to the supply of data voltage to the pixel circuits 100 arranged in different rows.
Hence, the conventional display device needs to adopt a structure where electric signals can be supplied separately and independently to the pixel circuits 100 in different rows. Specifically, the low potential supplying line 107, the high potential supplying line 108, the scan line 109, the first control line 110, and the second control line 111 as many as the number of the rows in the matrix of the pixel circuits 100 are required. Each of the elements 107 to 111 is arranged to extend in a column direction from one end of an array substrate on which the pixel circuits 100 are arranged in a matrix to another end, in order to supply electric signals to all pixel circuits 100 in the same row.
Thus, the interconnection structure occupies extremely large area on the array substrate. As the area occupied by the interconnection structure increases, the area of the light-emitting surface of the OLED 101 decreases accordingly. Then the improvement in aperture ratio is difficult to achieve. On the other hand, if a single common line is provided as each of signal supplying lines such as the low potential supplying line 107 which supply the electric signals to the pixel circuits 100 arranged in different rows, the improvement in aperture ratio is allowed. However, such structure creates another problem, i.e., the level of the driving threshold voltage supplied by the driver element 102 fluctuates, for example, to deteriorate the display image quality.